Modern transmitters for applications such as cellular, personal, and satellite communications employ digital modulation techniques such as quadrature phase-shift keying (QPSK) in combination with code division multiple access (CDMA) communication. Shaping of the data pulses mitigates out-of-band emissions from occurring into adjacent channels but produces time-varying envelopes. In addition to amplifying individual waveforms with time varying envelopes, many transmitters (especially in base stations) are being configured to amplify multiple carriers. Multi-carrier signals have a wide distribution of power levels resulting in a large peak-to-average ratio (PAR). The operation of amplifiers (e.g., linear amplifiers) in these types of signals is very inefficient, since the amplifiers are sized so that their supply voltage is high enough to handle the large peak voltages even though the signals are much smaller a substantial portion of the time. Additionally, the size and cost of the power amplifier is generally proportional to the required peak output power of the amplifier.
Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Division Multiplexing (OFDM), and multi-carrier versions of Global Standard for Mobile Communication (GSM) and Code Division Multiple Access 2000 (CDMA 2000) are wireless standards and applications growing in use. Each requires amplification of a waveform with high PAR levels, above 10 dB in some cases. The sparse amount of spectrum allocated to terrestrial wireless communication requires that transmissions minimize out-of-band (OOB) emissions to minimize the interference environment. A linear amplifier used to amplify a waveform with a PAR of 10 dB or more provides only 5–10% DC-RF efficiency. The peak output power for the amplifier is sized by the peak waveform. The cost of the amplifier scales with its peak power. Several other circuit costs including heat sinks and DC-DC power supplies scale inversely to peak power and dissipated heat (which results from the electrical inefficiency). Related base station costs of AC-DC power supplies, back-up batteries, cooling, and circuit breakers also scale inversely with efficiency as does the electrical operating costs. Clearly, improving DC-RF efficiency is a major cost saver for both manufacture and operation.
One efficiency enhancement technique for power amplifiers is known as envelope tracking or envelope following. In an envelope tracking system, the supply voltage to a power amplifier is reduced or increased in response to the amplitude of the amplitude modulated envelope of the input signal. The supply voltage applied to the power amplifier follows the envelope of the input signal provided to the power amplifier. The supply voltage is maintained at levels that assure amplifier operation out of saturation. For example, when the envelope amplitude is at peak, the supply voltage is increased to a voltage greater than the desired amplifier output voltage at the signal peak. When the envelope amplitude is at its minimum, the supply voltage is decreased below the peak voltages, thus providing more efficient amplification than a constant supply linear amplifier.
Accurately tracking ensures that the supply voltage of the amplifier provides roughly a constant number of volts or “headroom” above the output power. Increasing this constant amount reduces efficiency but improves linearity. Conversely, reducing the difference between the supply voltage and the output voltage improves efficiency at the cost of increased distortion and out-of-band (OOB) emissions. Tracking an input signal envelope can require abrupt or high frequency changes in the supply voltage of the amplifier. Providing such drastic changes can require voltage supply components having a large bandwidth to properly supply the amplifier and avoid distortion of the amplifier output. Components having the desired bandwidth often have other undesired attributes, such as a lower efficiency than their low bandwidth counter parts. The efficiency loss from these components can partially or fully negate the efficiency gained by provided a variable amplifier supply.